Pneumatic percussion tools, such as pneumatic hammers, can comprise a cylindrical body provided with an internal bore in which a ram or piston is linearly reciprocated by the alternate supply of compressed air to chambers formed on opposite sides of the ram. The device is generally provided with a distributing valve system for this purpose and the ram functions as a double-acting piston which, on its forward stroke, is driven into engagement with an anvil or impact-receiving member.
Customarily, the impact-receiving member is a tool mounted in the end of the body of the apparatus and forming a rock-breaking chisel, a drill, a pick or the like. The tool can itself be axially slidable in or relative to the cylinder body.
A handle can be provided at the upper end of the pneumatic apparatus, depending upon the shape of the tool, the power and the stroke. The apparatus may be employed as a chisel for metal work or rock breakage, as a concrete breaker, as a drill or even as a ram for compaction purposes.
Obviously, when compressed air is fed to one of the compartments of the cylinder to drive the ram away from this compartment, the other compartment must be vented. Thermodynamic considerations require that the discharge of air from the compartments during the contraction of the volume thereof be as rapid and as complete as possible to allow for a high kinetic energy of the ram at the point of impact transfer and rapid cycling of the ram movements.
The most simple expedient for controlling the discharge of air is to use the piston at least in part as a valve member. Thus it has been proposed to provide the walls of the cylinder with one or more openings (orifices) which are selectively blocked and unblocked by the ram or piston as it is reciprocated within the cylinder. With proper dimensioning of the orifices, for example, the ram or piston can uncover a large venting cross section to permit the compartment with which this orifice communicates to be contacted rapidly by the extremely rapid discharge of air therefrom. The opening speed for the orifices is similarly rapid.
While such systems have proved to be efficient, the sudden discharge of several cubic decimeters of compressed air at a pressure of several bars, with correspondingly high velocity at practically explosive spontaneity, effected over periods of several thousands of a second and about a thousand times per minute, generates an extremely high noise level which is detrimental to the auditory system of the user or anyone in the vicinity of the operation of the pneumatic hammer.
Because of the natural desire of fabricators of such tools to provide quiet operating devices and the social legal and moral pressures in this direction, considerable effort has been expended in attempting to silence or muffle the discharge of air from pneumatic hammers.
While in most instances, efforts to muffle the discharge of the vented air interfere with the efficiency of operation of the pneumatic hammer, there is a system described in, for example, German patent document (Open Application-Offenlegungsschrift) DE-OS 23 49 296, which has been successfully used in many cases.
This system provides an annular receptacle which surrounds the ram cylinder of the apparatus and defined between an outer wall of the cylinder and an inner wall of an envelope or sheath or shell surrounding same, this receptacle or chamber having a volume sufficient to receive the air discharged from one of the working compartments but at a pressure which is not significantly greater than atmospheric pressure.
The air in this chamber, which can have a pressure significantly lower than 1 bar, can then be released into the atmosphere at a velocity significantly smaller than the velocity with which the chamber is charged, over a period which is the full duration of each stroke of the ram.
While this arrangement represents a major advance in the acoustic muffling of a pneumatic hammer, it does have the serious disadvantage that the efficiency of the discharge depends in large measure on the shape and cross section or area of the conduit through which the previously detained air within the chamber is discharged into the atmosphere.
The cross section of this conduit should be relatively small to permit storage at an elevated pressure albeit close to atmospheric, such that the outflow is practically continuous and not pulsed. The walls of the conduit should be as close together as possible to minimize the sound produced.
The storage of the compressed air, whether from the upper or lower compartment of the cylinder, in a chamber of the type described is always accompanied by a sharp drop in the temperature of the air as a result of the expansion to a lower pressure. Since the final temperature of the detained air is variable and is a function of ambient temperature, it can frequently drop below zero degrees C. as well as below the dewpoint of any moisture in the air.
Condensed or sublimed moisture, transformed into ice, can obstruct the discharge passage and the problem is especially pernicious with smaller cross sections or venting-passage widths.
The art has faced these problems in the past and the only solutions of which I am aware heretofore enlarged the cross section of the conduit by increasing its diameter and thereby increasing the noise evolved which is directly contradictory to the intent of the storage chamber in the first place. Attempts have also been made to use anti-icing systems and combinations of an anti-icing system with an increase in cross section. The prior techniques have all been found to be unsatisfactory and to limit the efficiency of the apparatus or to be uneconomical.